1) Field of the Invention
The present invention relates to a transport systems and a method of monitoring a burst error in a synchronous optical network (SONET), and more particularly, to a transport systems and a method of monitoring a burst error that detects the burst error in a transmission signal having header information and data information multiplexed into a frame in bytes, and that notifies a line failure or executes line switching.
2) Description of the Related Art
A SONET transport systems monitors an error in a transmission signal by monitoring a parity using B2 byte of a line overhead (LOH) included in STS-N (N=1, 3, 12, 48, 192) frame.
FIG. 7 is a block diagram of an interface circuit of a transport systems. An optical/electrical (O/E) converter 701 provided in the interface circuit 700 converts an optical signal to be transmitted into an electrical signal, and outputs data and a clock. A serial-to-parallel (SP) processing unit 702 performs serial-to-parallel conversion of the data and the clock, and outputs parallel data and clock to a termination unit 703. The termination unit 703 detects a line error and switches a working line to a protection line or notifies an occurrence of a line error to outside based on the line error detected so as to repair a line failure. When a problem of degradation of amplitude of the clock occurs, error notification and line switching for reducing line damages can be carried out in this manner. One of the popular techniques to detect the error in a signal to be transmitted is to perform a parity check of a bit error (for example, see Japanese Patent Application Laid-open No. S52-9335).
FIG. 8 is a block diagram of a conventional termination unit. The termination unit 703 is provided with a signal failure detecting unit 801 that detects a line failure, a signal degradation detecting unit 802, an OR circuit 803, and an ACT (Activation)/DEACT (Deactivation) processing unit 804. The signal failure detecting unit 801 detects loss of signal (LOS), loss of frame (LOF), alarm indication signal-Line (AIS-L), and B2-Major (B2-MJ). The signal degradation detecting unit 802 detects B2-Minor (B2-MIN).
Detection signals from the signal failure detecting unit 801 and signal degradation detecting unit 802 are output to the ACT/DEACT processing unit 804 via the OR circuit 803. The ACT/DEACT processing unit 804 outputs a processing signal indicating a line failure after a predetermined guard time measured by an internal timer has elapsed since inputting of the detection signals. A notification of a line failure and a request for line switching are output as the processing signal. Based on the request for the line switching, a switching from the working line to the protection line is performed.
However, a generation of a burst error in the transport systems is assumed on application of surge or burst electrostatic discharge (ESD), such as lightening, and a line error that occurs at the time of line switching caused by a failure in machine. In general, the transport systems is supposed to suppress an unnecessary line switching of a normal portion when the burst error occurs.
Therefore, even if the burst error is generated based on a line failure originated from a failure in the conventional transport systems, it is not possible to notify the occurrence of the burst error or switch the working line where a burst error has occurred to the protection line.
Although the degradation of the clock that is cause of the burst error can also be considered as a sign of a bit error, it is generally detected as out of frame (OOF) based on a change in the clock count number of an internal circuit (logic circuit) that constitutes the SP processing unit 702.
FIG. 9A and FIG. 9B are schematic diagrams for illustrating a generation of the burst error: FIG. 9A depicts a case of clock degradation; and FIG. 9B depicts a case of data degradation. In a normal mode, a position f of the data always coincides with a position of the clock count. When the clock is degraded, however, one clock is dropped out, and the frame top position f of the data is shifted from the position of the clock count. This is the state of OOF. When the OOF occurs, the internal circuit of the SP processing unit 702 performs resynchronization but the data over the period up to the resynchronization keeps generating data errors, leading to a state where a burst error occurs. According to a specification of the SONET, OOF, it is regulated to perform the line switching as LOF when the OOF is sustained for 3 milliseconds; however, the OOF alone does not bring about the line switching.
FIG. 10 is a schematic diagram for illustrating the STS-N frame format. The OOF is defined as being out of synchronization over five consecutive frames (125×5=625 microseconds). Accordingly, the number of error bits caused by the burst error can be defined as 32,400×N bits for the STS-N (N=1, 3, 12, 48, 192) frame. Note that 32,400 bits=90 bytes×9 bytes×8 bits×5 frames.
FIG. 11 is a graph of a burst error rate characteristic for STS-48 (N=48). The horizontal axis in the diagram represents an interval (minute) of occurrence of the burst errors and the vertical axis represents an error rate. Even when one burst error occurs every ten minutes, the burst error occurs at a considerably high error rate of 8.3E-03, and thus it cannot be neglected. Generally, the error rate of 1.00E-03 is used as a set value for B2-MJ and the error rate of 1.00E-06 is used as a set value for B2-MIN.
Data is identified by a difference from a reference value. At this time, when a noise component is superposed to a reference value R and the level of the identification value becomes about the same level as the reference value R (corresponding to 1, 2, and 3 in the diagram), the noise frequency generated then may lead to erroneous detection of the identification values 1, 2, and 3 all being “0”, resulting in a burst error. As described above, conventionally, the burst error that is generated discretely due to the line failure or the machine failure cannot be detected as a burst error.
The line error is monitored normally by comparing a result of vertical computation of data after descrambling all bits of the STS-N frame (frame 1) excluding three rows of the unit overhead (SOH) (the shaded range in FIG. 10A) with B2 of the next frame (Frame 2) after descrambling. Specifically, the method called Bit Interleaved Parity N×8 (BIP-N×8) divides entire information to be subjected to error monitoring into groups of N×8 bits and monitors an error group by group. In the comparison of the count result with an even parity, it is possible to detect 0 (no error) with even number of 1's, and 1 (error occurred) with odd number of 1's. Note that the number of bits of B2 increases as 8 bits for STS-1 and 8×3=24 bits for STS-3.
In the example of STS-48, the total number of bits in one frame (125 microseconds) becomes 9 bytes×90 bytes×48 (equivalent to N)×8 bits=311,040 bits. Even when noise is superposed to the reference value R and a data identification error occurs in one frame, causing the burst error, it is not possible to detect more than 384 bits (N=48×8 bits) at a maximum with the parity error of B2 alone.
FIG. 12 is a graph for illustrating a parity saturation characteristic when monitoring B2 parity. The horizontal axis represents the bit error rate and the vertical axis represents probability of occurrence of parity errors. From the diagram, the B2 parity saturation characteristic is 0.5 (when the bit error rate is 1.28E-03 or greater). This means that the B2 parity error detectable is merely about 192 bits, half the maximum of 384 bits. Because such parity monitoring (even parities) makes an error of two bits or more appear just as a 1-bit error or 0-bit error, it is possible that even when the count value of B2 is “0”, the burst error occurs.
FIG. 13 is a table of a list of detection conditions for a B2 error in the STS-48. The bit error monitoring can detect the bit error as a random error that can be detected under any one of the normal error monitoring conditions as given in the B2 error detection conditions in the diagram. However, the clock degradation and bit error (burst error) originated from the data identification error could not be detected as the random error because of the large number of errors (the saturation region of the B2 parity) and discontinuity.
The burst error that occurs due to the line failure should be considered as a decision condition under which the occurrence of a line failure is to be detected because of considerable number of errors. However, the conventional technology has a difficulty in detecting discrete burst errors and cannot include the burst error in the conditions for determining a line failure, and currently there is no effective way to monitor the occurrence of the data identification error and the burst error originated from clock degradation.